Method For Trapping Carbon Dioxide By Cryocondensation

ABSTRACT

The present invention relates to a method of capturing carbon dioxide in a fluid comprising at least one compound more volatile than carbon dioxide CO2, for example oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.

The present invention relates to a method of capturing carbon dioxide ina fluid comprising at least one compound more volatile than carbondioxide CO2, for example oxygen O2, argon Ar, nitrogen N2, carbonmonoxide CO, helium He and/or hydrogen H2.

The invention can be notably applied to units producing electricityand/or steam from carbon fuels such as coal, hydrocarbons (natural gas,fuel oil, petrochemical residue, etc), household waste, biomass but canalso be applied to gases from refineries, chemical plants, steel-makingplants or cement works. It could also be applied to the flue gases fromboilers used to heat buildings or even to the exhaust gases fromtransport vehicles, and more generally to any industrial process thatgenerates CO2-containing flue gases.

Carbon dioxide is a greenhouse gas. For environmental and/or economicreasons, it is becoming increasingly desirable to reduce or eveneliminate discharges of CO2 into the atmosphere by capturing it andthen, for example, storing it in appropriate geological layers or byrealizing it as an asset in its own right.

A certain number of techniques for capturing carbon dioxide, for examplemethods based on scrubbing the fluids with solutions of compounds thatseparate the CO2 by chemical reaction, for example scrubbing using MEA,are known. These methods typically have the following disadvantages:

-   -   high energy consumption (associated with the regeneration of the        compound used to react with the CO2),    -   degradation of the compound that reacts with the carbon dioxide,    -   corrosion due to the compound reacting with the carbon dioxide.

In the field of cryo-condensation, that is to say of cooling until solidCO2 appears, mention may be made of document FR-A-2820052 whichdiscloses a method allowing CO2 to be extracted by anti-sublimation,that is to say by solidification from a gas without passing via theliquid state. The cold required is provided by means of fractionateddistillation of refrigerating fluids. The fluid containing CO2 and atleast one compound more volatile than CO2 is separated by alternatecondensation in two vessels. While the CO2 is condensing in one vessel,the CO2 is subliminated in the other, raising the pressure to the triplepoint for CO2 before the liquid CO2 thus created is extracted. Thismethod has the following disadvantages:

-   -   the exchangers have to be able to withstand pressures in excess        of 5 bar,    -   the capture efficiency is degraded because of the thermal        inertia of the system during the transitions between cycles,    -   large exchange areas are needed, making the equipment more        bulky,    -   valves are needed to switch from one vessel to another, and this        can reduce the reliability of the system.

Document US 2005/072186 describes a method for purifying natural gascontaining CO2. The mixture is cooled and solid CO2 is formed within theliquefied natural gas. A “soup” of solid CO2 and LNG is extracted via anendless screw, then heated up to liquefy the CO2. One disadvantage ofthis method is that a certain amount of liquefied natural gas isextracted at the same time as the solid CO2, and this then has to berecirculated. Further, the CO2 forms blocks of solid which are not easyto extract.

It is one object of the present invention to provide an improved methodof capturing carbon dioxide from a fluid containing CO2 and at least onecompound more volatile than the latter.

The invention relates first of all to a method for producing at leastone CO2-lean gas and one or more CO2-rich fluids from a process fluidcontaining CO2 and at least one compound more volatile than CO2, saidmethod implementing:

a) a cooling in at least one vessel of at least part of said processfluid so as to obtain at least one solid containing predominantly CO2 bycryo-condensation of part of the process fluid and at least one residualgas containing said CO2-lean gas;b) an extraction from said vessel of at least part of said solid formedin step a); andc) a liquefaction and/or sublimation of at least a part of said solidextracted in step b) so as to obtain one or more CO2-rich fluids;characterized in that the time intervals during which said step b) isimplemented are included within the time intervals during which saidstep a) is implemented.

The process fluid generally comes from a boiler or any plant thatproduces flue gases. These flue gases may have undergone variouspre-treatments, notably with a view to removing NOx (oxides ofnitrogen), dust, SOx (oxides of sulfur) and/or water.

Prior to separation, the process fluid is either monophasic, in gaseousor liquid form, or polyphasic. What is meant by “gaseous” form is“essentially gaseous” form, that is to say that it may notably containdust, solid particles such as soot and/or droplets of liquid.

The process mixture contains CO2 that is to be separated from the otherconstituents of said fluid by cryo-condensation. These otherconstituents comprise one or more compounds more volatile than carbondioxide in terms of condensation, for example oxygen 02, argon Ar,nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2. Theprocess fluids generally comprise predominantly nitrogen orpredominantly CO or predominantly hydrogen.

In step a) the process fluid is cooled in at least one vessel. Thiscooling may advantageously take place at least in part by exchange ofheat with CO2-rich fluids from the separation process. In addition or asan alternative, it may take place at least in part by exchange of heatwith the CO2-lean gas from the separation process. These cold fluidsfrom the separation process are heated up, while the process fluid iscooled down. This makes it possible to reduce the amount of energyrequired for the cooling operation.

As the cooling continues the initially gaseous CO2 is successfullysolidified by raising the process fluid to a temperature below thetriple point for CO2 while the partial pressure of the CO2 in theprocess fluid is below that of the triple point for CO2. This encouragesthe direct conversion of gaseous CO2 into solid CO2. For example, thetotal pressure of the process fluid is close to atmospheric pressure.This solidification operation is sometimes known as “cryo-condensation”or “anti-sublimation” of the CO2 or, by extension, of the process fluid.

According to one particular embodiment, all the components of theprocess fluid which do not solidify in step a) or which are not lumpedtogether with the solid CO2, remain in the gaseous state. Theseconstitute the CO2-lean gas.

Certain compounds more volatile than CO2 do not solidify and remain inthe gaseous state. Together with the non-solidified CO2 these willconstitute said CO2-lean gas, that is to say will constitute said gasthat comprises less than 50% CO2 by volume and preferably less than 10%CO2 by volume. According to one particular embodiment, said CO2-lean gascontains less than 1% CO2 by volume. According to another particularembodiment, it contains more than 2% thereof. According to anotherparticular embodiment, it contains more than 5% thereof. A solidcomprising predominantly CO2, that is to say containing at least 90% byvolume if considered in the gaseous state, preferably containing atleast 95% by volume, and more preferably still containing at least 99%CO2 by volume, is formed.

This solid may comprise other compounds than CO2. Mention may, forexample, be made of other compounds which might also have solidified, oralternatively of bubbles and/or drops of fluid contained within saidsolid lump. This explains how the solid could potentially consist of notonly solid CO2. This “solid” may contain non-solid parts such as fluidinclusions (drops, bubbles, etc).

In step b), at least part of this solid is extracted from the vessel inwhich the cryo-condensation took place. This extraction may be achievedwith no specific action, because of the shape of the vessel in which thecryo-condensation takes place, or may occur through the intervention ofdedicated means. The solid extracted is possibly transported to othervessels.

The inventors have demonstrated that is particularly advantageous forenergy reasons, but also for mechanical reasons, for step b) to becarried out during time intervals contained within the time intervals inwhich step a) is implemented.

What is meant by a time interval during which a certain step E iscarried out, is the duration comprised between an instant t1 in whichthe step E commences and an instant t2 at which it ends, without therebeing any interruption in the step E between t1 and t2. A time interval[t1; t2] is said to be comprised or contained in a time interval [t3;t4] if t3 occurs before or at the same time as t1 and t4 occurs after orat the same time as t2.

Stated differently, when step b) is implemented, step a) is in theprocess of being performed. When the solid-extraction operations in stepb) occur, the operations of cooling the process fluid andcryo-condensation also occur. This naturally assumes that thecryo-condensation and the extraction relate to different molecules.

Next, in step c), the solid is returned to temperature and pressureconditions such that it changes into a fluid, liquid and/or gaseous,state. At least part of said solid may then liquefy (melt). This thengives rise to one or more CO2-rich primary fluids. These fluids are saidto be “primary” to distinguish them from treatment fluids which are saidto be “secondary”. What is meant by “CO2-rich” is something “comprisingpredominantly CO2” within the meaning defined hereinabove.

It is moreover advantageous to carry out the first and/or the secondcooling of the process fluid using one or more refrigerating cycles eachcomprising at least one near-isentropic expansion of a gas. Theserefrigerating cycles consist of several steps which cause a so-called“working” fluid to pass via several physical states characterized bygiven composition, temperature, pressure, etc conditions. The presence,among the steps of the cycle, of at least one near-isentropic expansion,that is to say of an expansion that causes the entropy of the expandedfluid to increase by less than 25%, preferably less than 15% and morepreferably still, less than 10% makes it possible to improve the energyconsumption of the separation process.

Depending on circumstances, the method according to the invention maycomprise one or more of the following features:

said process fluid is essentially gaseous.

said solid containing predominantly CO2 and formed in step a) andextracted in step b) is in the form of carbon dioxide snow. It thereforehas the consistency of snow, making it easier to extract.

said cryo-condensation occurs through deposition on one or moresurfaces. said surfaces are the internal and/or external surfaces oftubes.

said surfaces are the external surfaces of solid particles.

said surfaces are oriented in such a way that at least part of saidsolid periodically falls off under the effect of gravity once a certainthickness of said solid has built up on said surfaces.

said surfaces are periodically scraped to remove at least part of saidsolid progressively as it forms.

said scraping is performed at least in part by one or more endlessscrews.

at least part of said surfaces is given a vibrating movement that helpsto dislodge at least part of said solid.

at least part of said surfaces is periodically heated in order to detachat least part of said solid and cause it to fall off.

said cryo-condensation of said solid takes place on supporting solidparticles that form a fluidized bed.

said supporting solid particles on which said solid is condensed in afirst reactor are removed from said first reactor then regenerated in asecond reactor in order to rid them of at least part of said solid whichthey support.

said supporting solid particles are taken from said first and secondreactors by gas-solid separation in cyclones.

said supporting solid particles contain at least one metal and/or oneplastic or alternatively contain predominantly CO2.

at least part of said solid is extracted in said step b) under theaction of one or more endless screws.

at least part of said solid is dislodged from said surfaces or from saidsupporting solid particles on which said solid is condensed in step a),said dislodging being obtained under the effect of pressure waves orassisted by pressure waves.

A solid forms and adheres to the walls of the vessel in which theprocess mixture is cooled and cryo-condensed.

These surfaces may be of variable shapes. They may be flat or twisted.According to one particular embodiment the geometry of the vessel istubular, that is to say that the process fluid circulates through hollowtubes and/or around hollow or solid tubes. According to anotherembodiment, cryo-condensation occurs at the surface of particles ofvarying shapes, for example beads.

The solid which forms on these surfaces can be extracted in variousways. If the surfaces or the particles on which cyro-condensation occursare mobile, then it is not strictly necessary to dislodge the solid. Ifthese surfaces do not leave the vessel in which cryo-condensationoccurs, then it becomes necessary to detach the solid and transport itout of said vessel.

According to one particular embodiment, the surfaces in question areoriented in such a way that the solid can fall off under its own weightwhen a certain thickness has built up. Said surfaces may also be scrapedby any mobile means of a shape suited to said surfaces. According to oneparticular embodiment, scraping may be performed by one or more endlessscrews positioned near said surfaces or in contact with said particles,at a distance such that the screw bites into or displaces the layer ofsolid that is to be extracted.

These surfaces, whatever they might be, may also be given a vibrationalmovement causing or encouraging the solid to be dislodged. The frequencyand amplitude of these movements will be tailored so that detachmentoccurs for a certain thickness of solid.

Said surfaces may also be heated to detach all or some of the solid.According to one particular embodiment, this heating is performed byelectrical tracing, that is by running heating electrical resistorsthrough the structure of the vessel.

According to an alternative of the method, cryo-condensation takes placeon particles in a fluidized bed. These particles are circulated from theregions in which cryo-condensation takes place towards regions in whichthe particles lose at least some of the layer of solid formed at theirsurface. One possible embodiment is to have one or morecryo-condensation reactors and one or more regeneration reactors betweenwhich reactors said particles circulate. These particles are generallyseparated from the gaseous streams by cyclones. Regeneration of theparticles may or may not include re-cooling of said particles to atemperature of below the triple point temperature of CO2.

These particles may contain various materials, particularly metal and/orplastic. They may comprise solid containing predominantly CO2. In oneembodiment, they grow or even appear in the cryo-condensation reactorand diminish, or even disappear, in the regeneration reactor.

The solid that has become detached may be transported away from thevessel in which cryo-condensation has occurred using one or more endlessscrews.

According to another embodiment, the solid may be detached from saidsurfaces by sending pressure waves, for example ultrasonic waves, intothe cryo-condensation vessel.

All of the aforementioned means for detaching the solid deposited onsaid surfaces can be implemented alone or in combination.

The invention also relates to the method applied to industrial fluegases with a view to capturing CO2.

According to one particular embodiment, these flue gases come from aplant producing energy (steam, electricity) and may have undergonepretreatments.

The invention will be better understood upon reading the description andexamples that follow, which are non limiting. They make reference to theattached drawings in which:

FIG. 1 schematically depicts a CO2 capture unit implementing a methodaccording to the invention, FIG. 2 schematically depicts thecryo-condensation vessel of a plant implementing a method according tothe invention, FIG. 3 schematically depicts the use of a methodaccording to the invention in a plant for producing electricity fromcoal.

The plant illustrated in FIG. 1 implements the steps described below.the fluid 24 consisting of flue gases is compressed in a compressor 101,notably to compensate for the pressure losses in the various pieces ofequipment in the unit. Let us note that this compression may also becombined with the compression known as the draft compression of theboiler that produces the flue gases. It may also be carried out betweenother steps of the method, or downstream of the CO2 separation method;the compressed fluid 30 is injected into a filter 103 to eliminateparticles down to a level of concentration of below 1 mg/m³, preferablyof below 100 μg/m³; next, the dust-free fluid 32 is cooled to atemperature close to 0° C., generally of between 0° C. and 10° C., so asto condense the water vapor it contains. This cooling is carried out ina tower 105, with water injected at two levels, the cold water 36 andwater 34 at a temperature close to ambient temperature. It is alsopossible to conceive of indirect contact. The tower 105 may or may nothave packings; the fluid 38 is sent to a unit that eliminates residualwater vapor 107, for example using one and/or another of the followingmethods:

adsorption on fixed beds, fluidized beds and/or rotary dryer, theadsorbent potentially being activated alumina, silica gel or a molecularsieve (3A, 4A, 5A, 13X, . . . );

condensation in a direct-contact or indirect-contact exchanger. thedried fluid 40 is then introduced into the exchanger 109 where the fluidis cooled down to a temperature close to, but in all events higher than,the temperature at which CO2 solidifies. This temperature can bedetermined by a person skilled in the art aware of the pressure andcomposition of the process fluid 40. This temperature is situated ataround about −100° C. if the CO2 content of the process fluid is of theorder of 15% by volume and for a pressure close to atmospheric pressure,the fluid 42 which has undergone a first cooling 109 is then introducedinto a vessel 111 where it continues to be cooled down to thetemperature that provides the desired level of CO2 capture.Cryo-condensation of at least part of the CO2 contained in the fluid 42occurs producing, on the one hand, a CO2-lean gas 44 and, on the otherhand, a solid 61 comprising predominantly CO2. The gas 44 leaves thevessel 111 at a temperature of the order of −120° C. This temperature ischosen as a function of the target level of CO2 capture. At thistemperature, the CO2 content of the gas 44 is of the order of 1.5% byvolume, namely a capture level of 90% starting out from a process fluidcontaining 15% CO2. There are various technologies that can be used forthis vessel 111:

Continous solid cryo-condensation exchanger in which solid CO2 isproduced in the form of carbon dioxide snow, is extracted, for example,using a screw and pressurized to introduce it into a bath of liquid CO2121 in which a pressure higher than the triple point pressure for CO2obtains. This pressurization can also be carried out batchwise in asystem of silos. Continuous solid cryo-condensation may itself beperformed in various ways:

scraped surface exchanger, the scrapers for example being in the form ofscrews to encourage extraction of the solid;

fluidized bed exchanger so as to carry the carbon dioxide snow along andclean out the tubes using particles for example of a density greaterthan that of the carbon dioxide snow;

exchanger in which solid is extracted by vibration, ultrasound, apneumatic or thermal effect (intermittent heating so as to cause thecarbon dioxide snow to fall);

accumulation on a surface with periodic “natural” fall into a tank. thefluid 46 is then heated up in the exchanger 109. As it leaves, the fluid48 can also be used notably to regenerate the unit used for eliminatingresidual vapor (107) and/or for producing cold water (115) byevaporation in a direct-contact tower 115 into which a dry fluid 50 isintroduced which then becomes saturated with water, vaporizing some ofit; some of the cold required for the cryo-condensation carried out inthe vessel 111 is supplied by one or more cold sources (75). Likewise,some of the cold required for the first cooling 109 is supplied by oneor more cold sources (76); the solid 62 comprising predominantly CO2 istransferred to a bath 121 of liquid CO2; this bath 121 needs to beheated in order to remain liquid, to compensate for the addition of coldfrom the solid 62 (latent heat of fusion and sensible heat). This can bedone in various ways:

by exchange of heat with a hotter fluid 72,

by direct exchange, for example by tapping a fluid 80 from the bath 121,heating it in

the exchanger 109, and reinjecting it back into the bath 121. liquid 64comprising predominantly CO2 is tapped from the bath 121. this liquid issplit into three streams. In the example, the first is obtained by anexpansion 65 to 5.5 bar absolute producing a diphasic, gas-liquid, fluid66. The second, 68, is obtained by compression 67, for example to 10bar. The third, 70, is compressed for example to 55 bar. The 5.5 barlevel provides cold at a temperature close to the triple pointtemperature for CO2. The 10 bar level allows the transfer of the latentheat of vaporization of the fluid 68 at around −40° C. Finally, at 55bar, the fluid 70 does not vaporize during the exchange 109. There isefficient use to be made of the cold energy contained in the fluid 64during the exchange 109 while at the same time limiting the amount ofenergy required to produce a purified and compressed stream 5 of CO2;after the exchange 109, the primary fluids 66, 68, 70 are compressed toa pressure level higher than the critical pressure for CO2 using thecompressors 131, 132, 133.

FIG. 2 depicts a cryo-condensation vessel 200 kept cold notably byexchange with a fluid 75 that may be the working fluid of arefrigerating cycle. The process fluid 42, possibly pre-cooled, isintroduced into the vessel 200. As the fluid 42 is further cooled,cryo-condensation occurs, with a solid layer being deposited on the coldsurface 210. The horizontal orientation of this surface 210 means thatpart of the layer of solid 211 containing predominantly CO2 falls offfrom time to time. To prevent solid from building up in the bottom 212of the vessel 200, an endless screw 201 is used to extract the solid 62.The gas 44 which is CO2-lean, but still very cold, is used to cool thefluid 42 and/or to cause part of the cryo-condensation thereof.

FIG. 3 depicts a plant for producing the electricity from coal,employing various units 4, 5, 6 and 7 for purifying the flue gases 19.

A primary airflow 15 passes through the unit 3 in which the coal 15 ispulverized and carried along toward the burners of the boiler 1. Asecondary airflow 16 is applied directly to the burners in order toprovide additional oxygen needed for near-complete combustion of thecoal. Feed water 17 is sent to the boiler 1 to produce steam 18 which isexpanded in a turbine 8.

The flue gases 19 resulting from the combustion, comprising nitrogen,CO2, water vapor and other impurities, undergo various treatments toremove some of said impurities. The unit 4 removes the NOx for exampleby catalysis in the presence of ammonia. The unit 5 removes dust, forexample using an electrostatic filter, and the unit 6 is adesulfurization system for removing the SO2 and/or SO3. The units 4 and6 may be superfluous depending on the composition of the productrequired. The purified flow 24 from the unit 6 (or 5 if 6 is notpresent) is then sent to a low-temperature cryo-condensationpurification unit 7 to produce a relatively pure flow 26 of CO2 and anitrogen-enriched residual flow 25. This unit 7 is also known as a CO2capture unit and implements the method that forms the subject of theinvention, as illustrated, for example in FIGS. 1 and 2.

1-15. (canceled)
 16. A method for producing at least one CO2-lean gasand one or more CO2-rich fluids from a process fluid containing CO2 andat least one compound more volatile than CO2, comprising: a) a coolingin at least one vessel of at least part of said process fluid so as toobtain at least one solid containing predominantly CO2 bycryo-condensation of part of said process fluid and at least oneresidual gas containing said CO2-lean gas; b) an extraction from saidvessel of at least part of said solid formed in step a); and c) aliquefaction and/or sublimation of at least a part of said solidextracted in step b) so as to obtain one or more CO2-rich fluids;wherein the time intervals during which said step b) is implemented areincluded within the time intervals during which said step a) isimplemented.
 17. The method of claim 16, wherein said solid containingpredominantly CO2 and formed in step a) and extracted in step b) is inthe form of carbon dioxide snow.
 18. The method of claim 16, whereinsaid cryo-condensation occurs through deposition on one or moresurfaces.
 19. The method of claim 18, wherein said surfaces are theinternal and/or external surfaces of tubes.
 20. The method of claim 18,wherein said surfaces are oriented in such a way that at least part ofsaid solid periodically falls off under the effect of gravity once acertain thickness of said solid has built up on said surfaces.
 21. Themethod of claim 17, wherein said surfaces are periodically scraped toremove at least part of said solid progressively as it forms.
 22. Themethod of claim 17, wherein at least part of said surfaces is given avibrating movement that helps to dislodge at least part of said solid.23. The method of claim 17, wherein at least part of said surfaces isperiodically heated in order to detach at least part of said solid andcause it to fall off.
 24. The method of claim 16, wherein saidcryo-condensation of said solid takes place on supporting solidparticles that form a fluidized bed.
 25. The method of claim 24, whereinsaid supporting solid particles on which said solid is condensed in afirst reactor are removed from said first reactor then regenerated in asecond reactor in order to rid them of at least part of said solid whichthey support.
 26. The method of claim 25, wherein said supporting solidparticles are taken from said first and second reactors by gas-solidseparation in cyclones.
 27. The method of claim 25, wherein saidsupporting solid particles contain at least one metal and/or one plasticor alternatively contain predominantly CO2.
 28. The method of claim 16,wherein at least part of said solid is extracted in said step b) underthe action of one or more endless screws.
 29. The method of claim 16,wherein at least part of said solid is dislodged from said surfaces orfrom said supporting solid particles on which said solid is condensed instep a), said dislodging being obtained under the effect of pressurewaves or assisted by pressure waves.
 30. The method of claim 16, whereinit is applied to industrial flue gases with a view to capturing CO2.